What is ENUM?

The popular PSTN numbering mechanism uses numbers in the range 0-9 to indicate country, area, service, mobility and other details. For example, people in the USA know that numbers starting with 972 are located in the Dallas area and they also know what the cost of the service will be if they dial a number starting with these digits. Numbering plans explain the system that lies behind these numbers.

DNS names use alphanumeric characters to make it easier to remember the address. While it is hard to remember “+4930398330”, it is easier to remember “snom.com”.

While it would be easy to access a telephone with the number “sip:info@snom.com” from a computer with a QWERTY keyboard, dialling such a number from a normal telephone requires a finger-breaking procedure. ENUM solves this problem by providing a lookup mechanism that translates “+4930398330” into “sip:info@snom.com”.

While telephony is a primary application for ENUM, it’s not the only one. ENUM records may also contain addresses of Email, web pages or other Internet resources. However, this FAQ focuses on the telephony application for ENUM using snom phones.

How it works

ENUM follows a simple principle. To convert an E.164 number into a DNS number, just turn the number around, remove the leading “+” sign, insert a dot between the digits and add “e164.arpa” at the end. For example, “+4930398330” is converted to “0.3.3.8.9.3.0.3.9.4.e164.arpa”. This number is dialled using the standard SIP procedures.

However, in practice there are some obstacles. The SIP user-agent needs to be able to deal with DNS NAPTR and DNS SRV records. These records are a powerful way of redirecting requests to other domains. If there is no ENUM record available, the SIP user agent has to fall back on something more conservative, for example redirecting the call to a PSTN gateway.

Avoiding Cheating

When end users have the opportunity to redirect calls at the caller’s cost, they could redirect their calls to an expensive 900-number. When an SIP proxy redirects the call to such a number, callers might be unaware of the redirection and be in for a nasty surprise when they pay their bills at the end of the month. To avoid this danger, the SIP user agent needs to indicate the redirection. The caller can then decide whether to continue or terminate the call. Optionally, endpoints may explicitly ask for an ok on the redirection.

ENUM and Overlap Dialling

When you dial a number on a PSTN phone, you don’t usually have to press a dial button on the phone. Normally the network tells you when a number is complete and automatically gives you a ringback tone in this case. Unfortunately, this is not possible with ENUM. The phone has to know how long a number is.

This is relatively easy in countries with a fixed length dial plan like in North America, but in other countries it causes big problems. In these countries, users have to use SIP phones like cell phones: at the end of the dialling procedure, they have to press the Ok button to start the call.

For North America (country code 1), snom provides a special dial plan that automatically dials the number if it starts with a “1” and ten digits (for example, “19721234567” is converted to “+19721234567”). Numbers starting with “011” are automatically converted to their “+” notation (for example “0114930398330” is converted to “+4930398330”). Other numbers with seven digits are automatically converted to the area-specific numbers (for example “8310280” is converted to “+19728310280” if the area code is set to “972”).

For other areas, the phone automatically converts numbers starting with a “00” into the international syntax (for example “004930398330” is converted to “+4930398330”). National numbers are detected by a leading zero (for example “022621234” is converted to “+4922621234” if the country code is 49). Other numbers are interpreted as local numbers, for example “398330” is converted to “+4930398330” (country code 49, area code 30). In all these cases the user needs to press the Ok button in order to start the call. The phone will give a reminder with an “Ok?” question after the number.

How to turn ENUM on

snom VoIP phones support ENUM. By default, whenever you dial a number, the phone will first try to locate the ENUM entry for the number provided in the user part of the number (for example, if you dial “<sip:+4930398330@company.com>”). To make this dialling procedure easier, you should provide the phone with some information on your ENUM environment.

This description explains the procedure for the snom320. First of all, you need to go to the registration menu of the phone. You can do this by pressing F3 ("Reg") in the idle screen of the phone (see screenshot) or you press the “Settings” key of the phone and select the “Reg” item.

You will see your first registration on the screen. The star behind the registration number indicates that this is your active account, which is used for outgoing calls. With the “Next” button you can select another registration if you want to set up ENUM for another registration.

With the navigation key right , you get into the dial plan menu for this registration. Normally, there is no specific dial plan associated with the registration.

Press F4 ("Edit") to edit the dial plan. You will be asked if you want to use ENUM.

If you press “On”, the phone will ask you for your country. Enter your country code here (for example, 1 for North America or 49 for Germany), then press Ok.

After this you also need to specify the area code that you want to use. This code is necessary even if you want to make a local call.

After pressing Ok, the phone will show the new name of the dial plan in the registration menu. The first code behind the ENUM indicates your selected country and the number behind it is the area code.

Set up your own ENUM number

While it is relatively simple to use ENUM on the phone, setting up ENUM entries in the DNS requires some experience with DNS. Hopefully, with the increasing importance of ENUM, operators will offer simple front-ends to their DNS systems that allow easy change of DNS records.
snom is participating in the German ENUM trial and has reserved the +493039833 for ENUM trials. You can dial the same number that you would dial if you wanted to reach snom on the PSTN.
If you need more information, please take a look at the ENUM trials in Austria (http://enum.at), Korea (http://www.uriforum.or.kr/data2/data/GSC-8.pdf) or at tools like Managed DNS by AG Projects (http://ag-projects.com).